Important Linux Commands

Important Linux Commands

If you run ls without any additional parameters, the program lists the contents of the current directory in short form.

-l

Detailed list

-a

Displays hidden files

cp[options]source target

Copies source to target.

-i

Waits for confirmation, if necessary, before an existing target is overwritten

-r

Copies recursively (includes subdirectories)

mv[options]source target

Copies source to target then deletes the original source.

-b

Creates a backup copy of the source before moving

-i

Waits for confirmation, if necessary, before an existing targetfile is overwritten

rm[options]files

Removes the specified files from the file system. Directories are not removed by rm unless the option -r is used.

-r

Deletes any existing subdirectories

-i

Waits for confirmation before deleting each file

ln[options]sourcetarget

Creates an internal link from source to target. Normally, such a link points directly to source on the same file system. However, if ln is executed with the -s option, it creates a symbolic link that only points to the directory in which source is located, enabling linking across file systems.

-s

Creates a symbolic link

cd[options][directory]

Changes the current directory. cd without any parameters changes to the user’s home directory.

mkdir[options]directory

Creates a new directory.

rmdir[options]directory

Deletes the specified directory if it is already empty.

chown[options] username[:[group]]files

Transfers ownership of a file to the user with the specified username.

-R

Changes files and directories in all subdirectories

chgrp[options]groupnamefiles

Transfers the group ownership of a given file to the group with the specified group name. The file owner can change group ownership only if a member of both the current and the new group.

chmod[options]modefiles

Changes the access permissions.

The mode parameter has three parts: group, access, and access type. group accepts the following characters:

u

User

g

Group

o

Others

For access, grant access with + and deny it with –.

The access type is controlled by the following options:

r

Read

w

Write

x

Execute—executing files or changing to the directory

s

Setuid bit—the application or program is started as if it were started by the owner of the file

As an alternative, a numeric code can be used. The four digits of this code are composed of the sum of the values 4, 2, and 1—the decimal result of a binary mask. The first digit sets the set user ID (SUID) (4), the set group ID (2), and the sticky (1) bits. The second digit defines the permissions of the owner of the file. The third digit defines the permissions of the group members and the last digit sets the permissions for all other users. The read permission is set with 4, the write permission with 2, and the permission for executing a file is set with 1. The owner of a file would usually receive a 6 or a 7 for executable files.

gzip[parameters]files

This program compresses the contents of files using complex mathematical algorithms. Files compressed in this way are given the extension .gz and need to be uncompressed before they can be used. To compress several files or even entire directories, use the tar command.

-d

Decompresses the packed gzip files so they return to their original size and can be processed normally (like the command gunzip)

taroptionsarchivefiles

tar puts one or more files into an archive. Compression is optional. tar is a quite complex command with a number of options available. The most frequently used options are:

-f

Writes the output to a file and not to the screen as is usually the case

-c

Creates a new tar archive

-r

Adds files to an existing archive

-t

Outputs the contents of an archive

-u

Adds files, but only if they are newer than the files already contained in the archive

-x

Unpacks files from an archive (extraction)

-z

Packs the resulting archive with gzip

-j

Compresses the resulting archive with bzip2

-v

Lists files processed

The archive files created by tar end with .tar. If the tar archive was also compressed using gzip, the ending is .tgz or .tar.gz. If it was compressed using bzip2, the ending is .tar.bz2.

locatepatterns

This command is only available if you have installed the findutils-locate package. The locate command can find in which directory a specified file is located. If desired, use wild cards to specify filenames. The program is very fast, because it uses a database specifically created for the purpose (rather than searching through the entire file system). This very fact, however, also results in a major drawback: locate is unable to find any files created after the latest update of its database. The database can be generated by root with updatedb.

updatedb[options]

This command performs an update of the database used by locate. To include files in all existing directories, run the program as root. It also makes sense to place it in the background by appending an ampersand (&), so you can immediately continue working on the same command line (updatedb &). This command usually runs as a daily cron job (see cron.daily).

find[options]

With find, search for a file in a given directory. The first argument specifies the directory in which to start the search. The option -name must be followed by a search string, which may also include wild cards. Unlike locate, which uses a database, find scans the actual directory.

Commands to Access File Contents

file[options][files]

With file, detect the contents of the specified files.

-z

Tries to look inside compressed files

cat[options]files

The cat command displays the contents of a file, printing the entire contents to the screen without interruption.

-n

Numbers the output on the left margin

less[options]files

This command can be used to browse the contents of the specified file. Scroll half a screen page up or down with PgUp and PgDn or a full screen page down with Space. Jump to the beginning or end of a file using Home and End. Press Q to exit the program.

grep[options]searchstringfiles

The grep command finds a specific search string in the specified files. If the search string is found, the command displays the line in which searchstring was found along with the filename.

-i

Ignores case

-H

Only displays the names of the relevant files, but not the text lines

-n

Additionally displays the numbers of the lines in which it found a hit

-l

Only lists the files in which searchstring does not occur

diff[options]file1file2

The diff command compares the contents of any two files. The output produced by the program lists all lines that do not match. This is frequently used by programmers who need only to send their program alterations and not the entire source code.

-q

Only reports whether the two files differ

-u

Produces a unified diff, which makes the output more readable

File Systems

mount[options][device]mountpoint

This command can be used to mount any data media, such as hard disks, CD-ROM drives, and other drives, to a directory of the Linux file system.

For hard disks not defined in the file /etc/fstab, the device type must also be specified. In this case, only root can mount it. If the file system needs to also be mounted by other users, enter the option user in the appropriate line in the /etc/fstab file (separated by commas) and save this change. Further information is available in the mount(1) man page.

umount[options]mountpoint

This command unmounts a mounted drive from the file system. To prevent data loss, run this command before taking a removable data medium from its drive. Normally, only root is allowed to run the commands mount and umount. To enable other users to run these commands, edit the /etc/fstab file to specify the option user for the relevant drive.

7.12.2 System Commands

The following section lists a few of the most important commands needed for retrieving system information and controlling processes and the network.

System Information

df[options][directory]

The df (disk free) command, when used without any options, displays information about the total disk space, the disk space currently in use, and the free space on all the mounted drives. If a directory is specified, the information is limited to the drive on which that directory is located.

-h

Shows the number of occupied blocks in gigabytes, megabytes, or kilobytes—in human-readable format

-T

Type of file system (ext2, nfs, etc.)

du[options][path]

This command, when executed without any parameters, shows the total disk space occupied by files and subdirectories in the current directory.

-a

Displays the size of each individual file

-h

Output in human-readable form

-s

Displays only the calculated total size

free[options]

The command free displays information about RAM and swap space usage, showing the total and the used amount in both categories.

-b

Output in bytes

-k

Output in kilobytes

-m

Output in megabytes

date[options]

This simple program displays the current system time. If run as root, it can also be used to change the system time. Details about the program are available in the date(1) man page.

Processes

top[options]

top provides a quick overview of the currently running processes. Press H to access a page that briefly explains the main options for customizing the program.

ps[options][process_ID]

If run without any options, this command displays a table of all your own programs or processes—those you started. The options for this command are not preceded by hyphen.

aux

Displays a detailed list of all processes, independent of the owner

kill[options]process_ID

Unfortunately, sometimes a program cannot be terminated in the normal way. In most cases, you should still be able to stop such a runaway program by executing the kill command, specifying the respective process ID (see top and ps). kill sends a TERM signal that instructs the program to shut itself down. If this does not help, the following parameter can be used:

-9

Sends a KILL signal instead of a TERM signal, bringing the specified process to an end in almost all cases

killall[options]processname

This command is similar to kill, but uses the process name (instead of the process ID) as an argument, killing all processes with that name.

Network

ping[options]hostname_or_IP address

The ping command is the standard tool for testing the basic functionality of TCP/IP networks. It sends a small data packet to the destination host, requesting an immediate reply. If this works, ping displays a message to that effect, which indicates that the network link is basically functioning.

-cnumber

Determines the total number of packages to send and ends after they have been dispatched (by default, there is no limitation set)

-f

flood ping: sends as many data packages as possible; a popular means, reserved for root, to test networks

-ivalue

Specifies the interval between two data packages in seconds (default: one second)

host[options]hostname[server]

The domain name system resolves domain names to IP addresses. With this tool, send queries to name servers (DNS servers).

ssh[options][user@]hostname[command]

SSH is actually an Internet protocol that enables you to work on remote hosts across a network. SSH is also the name of a Linux program that uses this protocol to enable operations on remote computers.

Miscellaneous

passwd[options][username]

Users may change their own passwords at any time using this command. The administrator root can use the command to change the password of any user on the system.

su[options][username]

The su command makes it possible to log in under a different username from a running session. Specify a username and the corresponding password. The password is not required from root, because root is authorized to assume the identity of any user. When using the command without specifying a username, you are prompted for the root password and change to the superuser (root). Use su – to start a login shell for a different user.

halt[options]

To avoid loss of data, you should always use this program to shut down your system.

reboot[options]

Does the same as halt except the system performs an immediate reboot.

clear

This command cleans up the visible area of the console. It has no options.